In order to delete a user from the approval workflow, please follow the steps below:
Go to the left side of the menu and choose Configuration.
Choose Approval workflow for Purchase request/Purchase order/invoice/Warehouse Request/receipt.
Firstly, add a new approver for the required step (instead of the user, you want to remove).
Choose the desired user that you want to remove from the approval workflow.
Click on X sign in the right upper corner.